BIOCHEMISTRYBIOCHEMISTRYUNIT 1UNIT 1

PART 4 EnzymesPART 4 Enzymes

ENZYMESLiving systems depend on reactions that

occur spontaneously, but at very slow rates.

Catalysts are substances that speed up reactions without being permanently altered.

No catalyst makes a reaction occur that cannot otherwise occur quick enough for life.

Most biological catalysts are proteins (enzymes); a few are RNA molecules (ribozymes).

In some exergonic reactions there is an energy barrier between reactants and products.

An input of energy (the activation energy or Ea) will put reactants into a transition state.

Enzymes lower the activation energy—they allow reactants to come together and react more easily.

Example: A molecule of sucrose in solution may hydrolyze in about 15 days; with sucrase present, the same reaction occurs in 1 second!

Enzyme-catalyzed reactions are part of metabolic pathways—the product of one reaction is a substrate for the next.

Homeostasis—the maintenance of stable internal conditions

Cells can regulate metabolism by controlling the amount of an enzyme.

Cells often have the ability to turn synthesis of enzymes off or on.

Medications (like Aspirin) can be INHIBITORS to enzyme action, causing a certain cellular process to stop (like increased blood flow).

DESCRIBE THE FUNCTION OF ENZYMES:

Enzymes are highly specific—each one catalyzes only one chemical reaction.

Reactants are substrates: they bind to a specific site on the enzyme—the active site.

Specificity results from the exact 3-D shape and chemical properties of the active site.

The enzyme–substrate complex (ES) is held together by hydrogen bonding, electrical attraction, or temporary covalent bonding.

The enzyme is not changed at the end of the reaction.

PEESSE

Binding of substrate to enzyme is like a baseball in a catcher’s mitt. The enzyme changes shape to make the binding tight: “induced fit.”

DESCRIBE THE STRUCTURE OF ENZYMES:

Some enzymes require ions or other molecules in order to function:

• Cofactors—inorganic ions

• Coenzymes – organic molecules, like vitamins and energy carriers can participate in many different reactions.

• Prosthetic groups (non-amino acid groups) permanently bound to their enzymes.

Rates of catalyzed reactions: There is usually less enzyme than substrate

present, so reaction rate levels off when the enzyme becomes saturated.

Saturated—all enzyme molecules are bound to substrate molecules.

Chemical inhibitors can bind to enzymes and slow reaction rates.

Natural inhibitors regulate metabolism; artificial inhibitors are used to treat diseases, kill pests, and study enzyme function.

Irreversible inhibition—inhibitor covalently binds to a side chain in the active site. The enzyme is permanently inactivated.

Reversible inhibition (more common in cells):A competitive inhibitor competes with natural

substrate for active site.A noncompetitive inhibitor binds at a site

distinct from the active site—this causes change in enzyme shape and function.

Allosteric regulation—an example of non-competitive reversible inhibition

non-substrate molecule binds a site other than the active site (the allosteric site)

The enzyme changes shape, which alters the chemical attraction (affinity) of the active site for the substrate.

Allosteric regulation can activate or inactivate enzymes.

EXAMPLES ARE PROTEIN KINASES

pH affects enzyme activity:Acidic side chains generate H+ and become anions.Basic side chains attract H+ and become cations.

Example:glutamic acid—COOH glutamic acid—COO– + H+

The law of mass action—the higher the H+ concentration, the more reaction is driven to the left to the less hydrophilic form.

This can affect enzyme shape and function.Protein tertiary structure (and thus function) is very

sensitive to the concentration of H+ (pH) in the environment.

All enzymes have an optimal pH for activity.

Temperature affects enzyme activity:Warming increases rates of chemical reactions, but

if temperature is too high, non-covalent bonds can break and inactivate enzymes.

All enzymes have an optimal temperature for activity.

DESCRIBE THE FOLLOWING FACTORS THAT AFFECT ENZYME ACTION:

Co-factors/co-enzymes/prosthetic groups

Enzyme saturation

Inhibition

pH

temperature